Circuit cards with conductors printed thereon have been manufactured by use of a subtractive method. In the subtractive method, copper foil is laminated onto a suitable substrate made of a synthetic material, and a photoresist is applied onto the copper foil. The photoresist is exposed and developed to produce the desired pattern of conductors by etching excess copper from the surface of the circuit card. As a result of this method, copper conductors can be made having widths larger than the thickness of the copper material laminated onto the circuit card. However, in view of the increasing pattern densities of circuit cards and the decreasing widths of the conductors currently used in the art, the subtractive method can no longer be utilized (owing to the lateral underetching of the photoresist masks).
With the ever increasing demand for the production of thin conductors, a number of additive metal deposition methods have been developed. In such an additive method, a thin copper foil is laminated onto the surfaces of of synthetic substrates. Instead of the unwanted copper being etched off, conductors are grown on the regions of the copper laminate which are not covered by a photoresist mask layer. Subsequently, the surplus copper of the laminated foil is etched off. The main problem inherent in such additive methods is providing a solid and reliable adhesion between the conductors applied by electroless deposition and the substrate of the circuit card.
Several adhesion-promoting methods known in the art will now be discussed in more detail.
One of the methods known in the art for increasing the adhesion between the conductive metal layers (which are applied by electroless deposition) and the substrate involves roughening the surface of the substrate to be coated by use of an abrasion process. Specifically, this method involves the steps of impressing a relief on a roughened surface; swelling and roughening the surface by means of acids, bases or solvents; using adhesion-promoting intermediate layers and embedding in the adhesion promoter foreign substances removable by means of acids or bases; or vapor depositing adhesion-promoting intermediate layers. It is also known (e.g. from German Offenlegungsschrift No. 24 45 803) to prepare the entire surface of a carrier plate by intensive, even repeated wet sand blasting.
German Offenlegungsschrift No. 24 25 223 discloses another method for improving the adhesion of metallic layers on the surface of a synthetic substrate. A solution of zinc oxide, copper (II) oxide, and sodium hydroxide is deposited on an aluminum foil to form a micro-nodular surface. The aluminum foil is then applied to the surface of the synthetic substrate. The aluminum foil and the zinc are removed in an etch bath, and copper conductors are applied by electroless plating onto the roughened surface of the synthetic substrate.
German Auslegeschrift No. 27 13 391 teaches a method for making a carrier material for printed circuits. In this method, the carrier material is coated with a thin copper layer and is transported through one or several roller pairs. A slurry is applied to the rollers or to the surface of the copper layer. The slurry contains quartz powder, glass powder or similar material. The rollers cause the surface of the copper layer to become micro-roughened by pressing the quartz or dust particles into the copper layer. A covering layer is then applied to the micro-roughened surface of the copper layer, and conductors are provided by metal deposition onto those areas of the copper layer not coated by the covering layer.
In more recent methods for making printed circuits, the copper is applied by vapor deposition in a vacuum or by sputtering onto laminates of synthetic material. These methods have not been used widely because the resulting thin copper layer has a relatively poor adhesion to the synthetic material laminate. Previously known roughening methods, such as those discussed above (as well as the roughening of epoxide resin surfaces in an oxygen plasma or the sand blasting process described in IBM Technical Disclosure Bulletin Vol. 25, No. 5, October 1982, p. 2339 by H. Meuller, J. Schneider, and F. Schwerdt), do not produce sufficient results because during the roughening process the thickness of the entire epoxide resin layer is reduced to a point where the embedded glass fiber fabric is adversely affected.
Finally, German Offenlegungsschrift No. 29 16 006 describes a method of making adhesive metallic layers on non-conductive surfaces, in particular on surfaces of synthetic material, wherein the surface regions to be coated are roughened by means of etching and are exposed prior to the etching process to a source of high energy radiation.
Thus, a need has arisen in the art for forming metallic layers on a non-conductive surface such as a synthetic material in which the metallic layers have a high adhesion characteristic (in the order of 1000 n/m) with respect to the non-conductive surface. As discussed above, the methods currently used in the art for increasing adhesion between a metallic layer and a synthetic substrate are unatisfactory where the metallic layer is either sputtered or deposited onto the substrate. Further, such adhesions have proved to be insufficient where the composite structure is to undergo further metallization steps in which thermal strains are applied. Moreover, the above-discussed bonding materials and adhesion promoters are expensive, and their uniform application involves considerable effort.